The present invention concerns a method for determining, inter alia, the hydride content in a measurement object by means of an inductive eddy current method. Methods and device for determining layer thickness by means of eddy currents are known through U.S. Pat. No. 5,017,869 and UK-A-2187844.
The published Swedish patent application 9602658-8 (and the corresponding U.S. patent application Ser. No. 08/679 624, Jourdain et al) describes a method and a device for determining the thickness of one or more layers arranged on each other on a substrate, wherein at least one of the layers or the substrate is electrically conductive. According to this method (and device) an electromagnetic alternating field is generated in the immediate vicinity of the outermost layer such that this field generates eddy currents in said electrically conductive layer, which eddy currents retroact on this field, and the influence of the eddy currents on the alternating field is measured and the thickness of the layer or layers is determined based on this measurement. Said measurement is performed in that the frequency of the alternating field is set at at least two different values and the alternating field resulting from said retroaction is measured at these frequency values, wherein the thickness of the layer or layers is calculated on the basis of data obtained through the last mentioned measurement and information about at least some of the electromagnetic properties of the substrate and the layer or layers.
A method and a device of this kind may be used for measuring the thickness of one or more layers within different technical fields.
Said Swedish patent application describes primarily the use of such a method and of such a device for measuring the thickness of layers which are formed on substrates in the form of fuel rods in nuclear power reactors. Such fuel rods are usually formed by a zirconium alloy and comprise the fuel itself, usually in the form of a number of small sintered pellets of uranium dioxide. In the very reactive environment in which these fuel rods are located, a number of chemical reactions occur, of which one is that a layer of zirconium dioxide is formed on the zirconium alloy itself, wherein this layer grows inwards. This has as a consequence that the thickness of the substrate gradually decreases at the same time as the thickness of the oxide layer increases. Furthermore, a so-called crud layer is formed on the oxide layer. This layer usually consists of a mixture of Fe, Zn and O. Since the substrate is electrically conductive, when it consists of said zirconium alloy, the mentioned method may be used for determining the thickness of the oxide layer. It is the case that the eddy currents which are produced by the alternating field in the electrically conductive substrate will interfere with the alternating field itself, wherein this interference decreases with the distance between the coil and the substrate, i.e. with the thickness of the oxide layer and a possible crud layer position thereon. By the method and the device which are described in said Swedish patent application, it has been shown that a very good determination of layer thicknesses may be obtained.
Within different fields there may be a problem in that hydrogen penetrates into a material and changes the properties of the material. One case where hydrogen may impair the properties of a material is in a nuclear plant. For example, the cladding tubes of fuel rods in such a plant are, as has been mentioned, often produced in a zirconium alloy. In this zirconium alloy, hydrogen may penetrate and combine with zirconium to form zirconium hydride. This makes the material more fragile and more brittle. There is therefore a limit for how much hydrides may be permitted without the material becoming too brittle. The measurement of how much hydride there is in a substrate is particularly difficult if there may be one or more layers on the substrate, which is the case for example in connection with cladding tubes for fuel rods in a nuclear plant.
In order to measure the hydride content in the fuel rods it has previously been necessary to remove the fuel rods from the nuclear plant and to measure at another location. This procedure is complicated and expensive. It would therefore be desirable to find a simplified and improved method and device for measuring the hydride content of an electrically conductive substrate of a measurement object which also may have one or more layers positioned on the substrate.
The inventors of the present invention have surprisingly arrived at the fact that the method (and device) for measuring layer thicknesses which is described in the above-mentioned document, may be developed in order to also measure hydride content. The invention may be applied in any field where it is desirable to determine the hydride content in a substrate which may have one or more layers positioned on the substrate.
A method according to the invention for determining the hydride content in an electrically conductive substrate of a measurement object comprises the following steps:
generation of a first electromagnetic alternating field in the immediate vicinity of the measurement object, wherein the electromagnetic alternating field is such that it at least partly penetrates the substrate and in this substrate creates eddy currents which in their turn produce a second electromagnetic alternating field which retroacts on the first electromagnetic alternating field;
setting of the first electromagnetic alternating field at at least two different frequencies;
measurement at said frequencies of the combined electromagnetic alternating field which is formed by the interaction of the first and the second electromagnetic alternating fields; and
determination of the hydride content in the substrate by using data which have been obtained through said measurement and information about at least some of the electromagnetic properties of the measurement object.
According to an embodiment of the invention, said determination is done by means of a model, which describes a coil for the generation of the first electromagnetic alternating field and the constitution of the measurement object and the influence of the measurement object on the first electromagnetic alternating field generated by the coil. By using such a model, it has been shown that a good result in the determination of the hydride content may be achieved in a relatively simple manner. It should be noted that the model of the coil may be a simplified model where the coil for example is described as if it comprised a single turn of winding, or as if it comprised several turns of winding positioned in a plane.
According to another embodiment of the invention, the measurement object comprises at least one layer positioned on the substrate, wherein said determination comprises a calculation of a resulting combined electromagnetic alternating field, which calculation comprises the introduction of predetermined or known values for at least some of the electromagnetic properties of the substrate and/or layer or layers and the assumption of one or more free parameters, such as the electric conductivity of the substrate and the thickness of the layer or layers positioned on the substrate, and wherein said determination comprises an iterative process according to which the assumed free parameter or parameters are changed until the measured combined electromagnetic field corresponds to the is calculated field. By such an iterative process and such a calculation, it is possible to in a relatively simple manner arrive at the hydride content even if several unknown parameters have to be assumed in the determination.
According to another embodiment of the invention, the method also comprises the determination of the thickness of at least one layer positioned on the substrate, which determination comprises the use of data which have been obtained through said measurement and information about at least some of the electromagnetic properties of the substrate or the layers. It is an important advantage with the present invention that both the thickness of one or more layers positioned on the substrate and the hydride content in the substrate may be determined.
Still another embodiment of the invention comprises determination of the electric conductivity of the substrate, wherein from this determination the hydride content is determined. By first determining the electric conductivity, the hydride content may then be determined in a relatively simple manner.
The method according to the invention is particularly suited to be used in connection with a measurement object which comprises a structural element in a nuclear plant, wherein said substrate may be the material of the structural member and wherein there may be an oxide layer and a crud layer on said substrate. An example of such a structural element is as has been mentioned cladding tubes for the fuel rods. It may however be of interest to measure the hydride content also on other structural elements which are used in the reactive environment in a nuclear reactor. Such other structural elements comprise spacers, which, inter alia, hold the fuel rods at determined distances from each other, and casing tubes, so-called box walls, which surround a fuel assembly.
In order to achieve a good inductive coupling with the measurement object, the first electromagnetic alternating field in suitably generated by means of a coil which is positioned in the immediate vicinity of the measurement object.
According to still another embodiment of the invention, said coil is formed by an electrically conductive material and comprises at least a first spiral-shaped part which along essentially its full length is tangent to an essentially plane boundary surface. It has been shown that by such a coil, formed with a spiral-shaped part, a very high resolution may be obtained even when the measurement is done on a substrate with a coating of thin layers.
It has been shown to be advantageous if the measurements of the alternating field are performed over a wide frequency range extending over one or more orders of magnitude. Said frequency range may for example cover the frequencies between 500 kHz and 50 MHz.
A device according to the invention for determining the hydride content in an electrically conductive substrate of a measurement object comprises means for generating such a first electromagnetic alternating field in the immediate vicinity of the measurement object that this alternating field at least partly penetrates the substrate and in this substrate creates eddy currents which in their turn produce a second electromagnetic alternating field which retroacts on the first electromagnetic alternating field;
means for setting the first electromagnetic alternating field at at least two different frequencies;
means for measuring, at said frequencies, the combined electromagnetic alternating field which is formed by the interaction of the first and the second electromagnetic alternating fields; and
an arrangement arranged for determining the hydride content in the substrate by using data, which have been obtained through said measurement and information about at least some of the electromagnetic properties of the measurement object.
According to an embodiment of the device, said arrangement is arranged such that said determination is performed by means of a model which describes a coil for the generation of the first electromagnetic alternating field and the constitution of the measurement object and the influence of the measurement object on the first electromagnetic alternating field generated by the coil.
According to a further embodiment of the device, said arrangement has means for introducing predetermined or known values for at least some of the electromagnetic properties of the substrate and of on the substrate positioned layers, and means for introducing one or more free parameters, such as the electric conductivity of the substrate and the thickness of the layer or layers positioned on the substrate, wherein said arrangement is arranged such that said determination comprises a calculation of a resulting combined electromagnetic alternating field, which calculation uses the introduced predetermined or known values and the free parameters, and wherein the arrangement is arranged such that said determination comprises an iterative process according to which the assumed free parameter or parameters are changed until the measured combined electromagnetic field corresponds to the calculated field.
According to still another embodiment of the device, the arrangement is arranged such that also the thickness of at least one layer positioned on the substrate is determined by using data which have been obtained through said measurement and information about at least some of the electromagnetic properties of the substrate or the layers.
According to still another embodiment of the device, the arrangement is arranged such that the electric conductivity of the substrate is determined and such that from this determination the hydride content is determined.
According to an advantageous embodiment of the device, the device comprises a coil for generating said first electromagnetic alternating field. Said coil in formed of an electrically conductive material and comprises suitably at least a first spiral-shaped part which along essentially its full length is tangent to an essentially plane boundary surface.
The device may suitably be arranged such that the measurements of the alternating field are performed over a wide frequency range extending over one or more orders of magnitude.
The different embodiments of the device have advantages corresponding to those which have been described in connection with the embodiments of the method above
Concerning both the method and the device, the measurement is thus done at at least two different frequencies, for example at three or more different frequencies, preferably at five or more different frequencies. It has thus been shown that a good accuracy may be achieved if the measurement is done at several different frequencies.
It should also be noted that the present invention in useful whenever the hydride content is to be measured in different materials. Examples of such materials are zirconium alloys and hafnium alloys.